The application of a photocatalyst technology begins to be put to practical use by utilizing characteristics for promoting various chemical reactions such as the decomposition of environmental contaminated substances, malodorous components, miscellaneous bacteria, etc. As the examples thereof, there are exemplified antibacterial tiles used in the operating rooms of a hospital, filters of air cleaners or air conditioners, glass of illuminating lamps in a tunnel of an express-highway and the like.
While these examples show the practical uses employing an oxidation capability, there has been also carried out a study for obtaining hydrogen by making a photocatalyst act on water. However, the application of the photocatalyst is not limited to thereto, and the photocatalyst may be made to act on toxic substances to obtain useful chemical substances.
For example, it may be considered that the photocatalyst may be applied to a desulfurization process of crude oil.
FIG. 1 shows the desulfurization process of the crude oil which is generally currently carried out.
As shown in FIG. 1, when the crude oil is distilled, heavy naphtha is hydrogenated and refined to produce hydrogen sulfide from all sulfur contained in the crude oil and recover hydrogen sulfide. The hydrogen sulfide generated in this case is oxidized via a process called the Claus method (see FIG. 2) to recover sulfur. The Claus method indicates the process that ⅓ as much as the hydrogen sulfide is oxidized to produce sulfur dioxide gas and the sulfur dioxide gas is made to react with remaining hydrogen sulfide to obtain sulfur. In the process for producing sulfur, since not only the catalytic reaction of the sulfur dioxide gas and the hydrogen sulfide, but also a heating operation or an aggregation is repeated, enormous energy is required. Further, it is undesirably expensive to manage the sulfur dioxide gas.
In the above described process for hydrogenating the heavy naphtha, hydrogen gas is employed. FIG. 3 shows a hydrogen producing process which is ordinarily carried out.
FIG. 3 shows a hydrogen producing method called a low temperature hydrogen refining method or a nitrogen cleaning method. According to this method, hydrogen is produced from gas rich in hydrogen. This method can be also applied to crude gas generated from processes except the above described hydrocarbon decomposition process. In the hydrogen producing method, raw gas is compressed and the compressed gas is cleaned with sodium hydroxide to firstly remove carbon dioxide gas and hydrogen sulfide therefrom. Then, methane and hydrocarbon gas having C4 or more are cooled under a heat exchange between low temperature refined hydrogen gas and them in a heat exchanger to be liquefied and removed. Then, the liquefied and removed gas enters the bottom part of a nitrogen cleaning tower and is cleaned with liquefied nitrogen descending from the top of the tower on its way to the tower. Thus, liquid carbon monoxide and nitrogen are obtained from the bottom of the tower and refined and separated hydrogen gas is obtained from the top of the tower.
In these hydrogen producing processes, a refining process is needed for avoiding a catalyst from being made toxic mainly by a sulfur compound such as hydrogen sulfide. In addition, since a heating operation or an aggregation is repeated, enormous energy is required.
That is, sulfur is useful as a raw material for sulfuric acid or carbon disulfide and hydrogen gas to which an attention is recently paid as fuel for a fuel cell is used for the process for hydrogenating and refining the crude oil, a manufacture of a semiconductor, a modification of fat and oil, a welding, metallurgy and the like. However, in order to produce sulfur or hydrogen gas, a heating operation and an aggregation are repeated, so that extremely much energy is required.
As mentioned above, in the conventional sulfur producing process or the hydrogen producing process, the heating operation or the aggregation is repeatedly carried out, so that enormous energy is needed.
Therefore, if sulfur and hydrogen can be simply taken out from hydrogen sulfide and the taken-out hydrogen can be returned to a desulfurization process, this method will especially serve as an useful chemical recycle.